Multi-layer golf ball and method of making same

ABSTRACT

A golf ball including: a core, an inner cover layer formed directly over the core, the inner cover layer substantially consisting of ionomer and having a thickness of 0.05-0.10 inches and a Shore D hardness of 56-65, and an outer cover layer comprising an ionomer, having a Shore D hardness of at least 60, and being harder than the inner cover layer, wherein the golf ball has a PGA compression of no more than about 110 and a coefficient of restitution of at least 0.780, and the ratio of the diameter of the ball to the sum of the thickness of the inner cover layer and the thickness of the outer cover layer is less than 1911, and the sum of the thickness of the inner cover layer and the thickness of the outer layer is at least 0.09 inches.

FIELD OF THE INVENTION

The present invention generally relates to golf balls, and moreparticularly to a golf ball having a multi-layer ionomeric cover.

BACKGROUND OF THE INVENTION

Golf balls traditionally have been categorized in three differentgroups, namely as one-piece, two-piece and three-piece balls.Conventional two-piece golf balls include a solid resilient core havinga cover of a different type of material molded thereon. Three-piece golfballs traditionally have included a liquid or solid center, elastomericwinding around the center, and a molded cover. Solid cores of both twoand three-piece balls often are made of polybutadiene and the moldedcovers generally are made of natural balata, synthetic balata, orionomeric resins.

lonomeric resins are polymers containing interchain ionic bonding. As aresult of their toughness, durability and flight characteristics,various ionomeric resins sold by E. I. DuPont de Nemours & Company underthe trademark "Surlyn®" and by the Exxon Corporation (see U.S. Pat. No.4,911,451) under the trademark "Escor®" and the trade name "lotek", havebecome the materials of choice for the construction of golf ball coversover the traditional "balata" (transpolyisoprene, natural or synthetic)rubbers. The softer balata covers, although exhibiting enhancedplayability properties, lack the durability (cut and abrasionresistance, fatigue endurance, etc.) properties required for repetitiveplay. lonomeric resins are generally ionic copolymers of an olefin, suchas ethylene, and a metal salt of an unsaturated carboxylic acid, such asacrylic acid, methacrylic acid or maleic acid. Metal ions, such assodium or zinc, are used to neutralize some portion of the acidic groupin the copolymer, resulting in a thermoplastic elastomer exhibitingenhanced properties, i.e., durability, etc., for golf ball coverconstruction over balata.

While there are currently more than fifty (50) commercial grades ofionomers available from Exxon and DuPont, with a wide range ofproperties which vary according to the type and amount of metal cations,molecular weight, composition of the base resin (i.e., relative contentof ethylene and methacrylic and/or acrylic acid groups) and additiveingredients such as reinforcement agents, etc., a great deal of researchcontinues in order to develop a golf ball cover composition exhibitingthe desired combination of playability properties.

Golf balls are typically described in terms of their size, weight,composition, dimple pattern, compression, hardness, durability, spinrate, and coefficient of restitution (COR). One way to measure the CORof a golf ball is to propel the ball at a given speed against a hardmassive surface, and to measure its incoming and outgoing velocity. TheCOR is the ratio of the outgoing velocity to the incoming velocity andis expressed as a decimal between zero and one.

There is no United States Golf Association limit on the COR of a golfball but the initial velocity of the golf ball must not exceed 250±5ft/second. As a result, the industry goal for initial velocity is 255ft/second, and the industry strives to maximize the COR withoutviolating this limit.

U.S. Pat. Nos. 4,431,193 and 4,919,434 disclose multi-layer golf balls.U.S. Pat. No. 4,431,193 discloses a multi-layer ball with a hard innercover layer and a soft outer cover layer. U.S. Pat. No. 4,919,434discloses a golf ball with a 0.4-2.2 mm thick cover made from twothermoplastic cover layers.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a golf ball having asoft compression while maintaining good durability and a highcoefficient of restitution.

Another object of the invention is to provide a golf ball having a goodcoefficient of restitution as compared to other golf balls having asimilar compression and surface hardness.

A further object of the invention is to provide a large size golf ballhaving a soft compression and moderate spin rate.

Yet another object of the invention is to provide a soft golf ballhaving a moderate spin rate and good carrying distance while maintaininga relatively soft compression.

Another object of the invention is to provide a method of making a golfball having the properties described above.

The invention in a preferred form is a golf ball comprising a core, aninner cover layer comprising an ionomer and having a Shore D hardness ofabout 65 or less, and an outer cover layer comprising an ionomer andhaving a Shore D hardness which is greater than the Shore D hardness ofthe inner cover layer and is at least about 60. The sum of the thicknessof the inner cover layer and the thickness of the outer cover layer isat least 0.090 inches.

The golf ball of the invention preferably has a COR of at least 0.780,more preferably at least 0.790, and most preferably at least about0.800. In a particularly preferred form of the invention, the golf ballis an oversized ball having a diameter of at least 1.70 inches. The golfball has a weight in the range of 43.0-46.0 g. Preferably, the ratio ofthe diameter of the ball to the sum of the thickness of the inner coverlayer and outer layer is less than 19:1 and more preferably less thanabout 18:1. The PGA compression of the golf ball according to apreferred embodiment of the invention is no more than about 110, morepreferably no more than about 107 and even more preferably no more thanabout 100.

Another preferred form of the invention is a method for making amulti-layer golf ball. The method comprises the steps of (1) obtaining agolf ball core, (2) forming an inner cover layer over the core, theinner cover layer comprising an ionomer and having a Shore D hardness ofabout 60 or less and (3) forming an outer cover layer over the innercover layer, the outer cover layer having a Shore D hardness of at leastabout 60 and being harder than the inner cover layer. The sum of thethickness of the inner cover layer and the thickness of the outer coverlayer is at least 0.090 inches.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others and thearticle possessing the features, properties, and the relation ofelements exemplified in the following detailed disclosure.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a cross-sectional view of a golf ball according to apreferred embodiment of the invention.

FIG. 2 shows a diametrical cross-sectional view of the golf ball shownin FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The golf ball according to the invention has a central core and a thickcover which includes at least two ionomeric layers. The inner coverlayer is softer than the outer cover layer. This type of constructionresults in a golf ball having a favorable combination of softcompression and low spin while maintaining good COR.

Referring now to the drawings, a golf ball according to the invention isshown and is designated as 8. The ball preferably has a diameter of atleast 1.68 inches. The invention is particularly useful with golf ballshaving a diameter of 1.70 inches or more.

The golf ball includes a central solid core 10 and a multi-layer cover12. The multi-layer cover includes an inner cover layer 14, and an outercover layer 16 with dimples 18. The inner cover layer 14 comprises anionomer and is softer than the outer cover layer 16. The ionomer that isused to form the inner cover layer 14 can be a single type of ionomer ora blend of two or more types of ionomers. Hardening and/or softeningmodifiers can be blended with the ionomers. The inner cover layer has aShore D hardness of about 65 or less, more preferably 20-63 and mostpreferably 40-60 (ASTM D-2240).

In a preferred form of the invention, the inner cover layer comprises anionomer with an average wt % acid content of about 15 or less which isat least 10% neutralized. In a particularly preferred form of theinvention, the inner cover layer 14 constitutes a blend of two types ofionomers in which one component of the blend is an ethylene-acrylic acidor ethylene-methacrylic acid copolymer containing ≧15 wt % acid groupswhich are at least partially neutralized with a cation, and the othertype of ionomer is a terpolymer of ethylene, acrylic acid or methacrylicacid and a softening termonomer such as butyl acrylate or methylacrylate, resulting in an overall wt % acid content of about 15 or less.

The outer cover layer 16 also comprises an ionomer. The outer layer 16can contain a single type of ionomer or a blend of two or more types ofionomers. Furthermore, a hardening and/or softening modifier can beadded. The outer cover layer is harder than the inner cover layer, andhas a Shore D hardness of about 60 or more, more preferably 65 or moreand most preferably 68 or more (ASTM D-2240). In a particularlypreferred form of the invention, the outer cover layer contains anionomer having at least 16 weight % acid groups, which are at leastpartially neutralized.

The inner and outer layers together form a cover having a thickness of0.090-0.250 inches, more preferably about 0.10-0.20 inches, and mostpreferably about 0.11-0.15 inches. Preferably, the inner cover layer hasa thickness of 0.025-0.200 inches, and more preferably 0.050-0.100inches. The outer cover layer preferably has a thickness of about 0.025to 0.200 inches, or more preferably 0.050-0.100 inches. The ratio of thediameter of the ball to the thickness of the cover, i.e. the sum of thethickness of the inner cover layer and the outer cover layer, is no morethan about 19:1, preferably no more than about 17:1 and more preferablyno more than about 15:1.

The hardness of the inner and outer cover layers, and the compositionsand thicknesses of these layers are appropriate to result in a golf ballhaving a COR of at least 0.780, more preferably at least 0.790 and mostpreferably at least 0.800. The golf ball of the invention has an overallPGA compression of 30-110, more preferably 50-107, and most preferably60-90. Preferably, the spin rate of the golf balls is no more than about8000 revolutions per minute (rpm) when struck with a 9-iron underconditions of launch angle, ball speed and tee position which produce aspin rate of about 7100 rpm for a two-piece hard covered ball (1994Top-Flite XL) and a spin rate of about 9700 rpm for a thread woundbalata covered ball (1994 Titleist Tour 100) using the same club. Thisrelatively low spin rate provides the advantage of less sideways spinfor improved accuracy and less backspin for improved rolling distance.

The core of the golf ball preferably is made of a crosslinkedunsaturated elastomer and preferably comprises a thermoset rubber suchas polybutadiene, but also can be made of other core materials whichprovide sufficient COR. The diameter of the core is determined basedupon the desired overall ball diameter, minus the combined thicknessesof the inner and outer cover layers. The COR of the core is appropriateto impart to the finished golf ball a COR of at least 0.780, andpreferably at least 0.790. The core typically has a diameter of about1.0-1.6 inches, preferably 1.4-1.6 inches, a PGA compression of 80-100,and a COR in the range of 0.770-0.830.

Conventional solid cores are typically compression molded from a slug ofuncured or lightly cured elastomer composition comprising a high ciscontent polybutadiene and a metal salt of an α,β, ethylenicallyunsaturated carboxylic acid such as zinc mono or diacrylate ormethacrylate. To achieve higher coefficients of restitution in the core,the manufacturer may include fillers such as small amounts of a metaloxide such as zinc oxide. In addition, larger amounts of metal oxidethan those that are needed to achieve the desired coefficient are oftenincluded in conventional cores in order to increase the core weight sothat the finished ball more closely approaches the U.S.G.A. upper weightlimit of 1.620 ounces. Other materials may be used in the corecomposition including compatible rubbers or ionomers, and low molecularweight fatty acids such as stearic acid. Free radical initiators such asperoxides are admixed with the core composition so that on theapplication of heat and pressure, a complex curing cross-linkingreaction takes place.

The inner cover layer preferably comprises an ionomer having an averagewt % acid content of about 15 or less which is at least 10% neutralized.Preferably, the inner cover layer 14, includes a blend of hard and softionomer resins including but not limited to those described in U.S. Pat.Nos. 4,884,814 and 5,120,791, both of which are incorporated herein byreference. For example, a blend of a high modulus ionomer (i.e., a hardionomer) and a low modulus ionomer (i.e., a soft ionomer) in a mixturecan be used. A high modulus ionomer is one which has a flexural modulusof about 15,000-70,000 psi or more as measured under ASTM method D-790.The hardness of this type of ionomer is at least 50 on the Shore D scaleas measured in accordance with ASTM method D-2240. A low modulus ionomerwhich can be blended with the high modulus ionomer to form the innerlayer has a flexural modulus of about 1,000 to about 15,000 psi (ASTMD-790), and a hardness of about 20-40 on the Shore D scale (ASTMD-2240).

The hard ionomer resins used in blends to form the inner cover layer 14include ionic copolymers which are the e.g. sodium, zinc, magnesium,calcium, manganese, nickel, potassium or lithium, etc. salt, or blendthereof, of the reaction product of an olefin having from 2 to 8 carbonatoms and an unsaturated monocarboxylic acid having from 3 to 8 carbonatoms and which result in a ball having the desired combination of COR,compression, and inner cover layer hardness. The carboxylic acid groupsof the copolymer may be totally or partially, that is 10-100%,neutralized. The hard ionomeric resins typically are copolymers ofethylene with acrylic and/or methacrylic acid. Two or more hard ionomerresins can be blended.

The metal cation salts utilized in the invention are those salts whichprovide the metal cations capable of neutralizing, to various extents,the carboxylic acid groups of the high acid copolymer. These includeacetate, oxide or hydroxide salts of e.g. lithium, calcium, zinc,sodium, potassium, nickel, magnesium, and manganese, etc.

Examples of such lithium ion sources are lithium hydroxide monohydrate,lithium hydroxide, lithium oxide and lithium acetate. Sources for thecalcium ion include calcium hydroxide, calcium acetate and calciumoxide. Suitable zinc ion sources are zinc acetate dihydrate and zincacetate, a blend of zinc oxide and acetic acid. Examples of sodium ionsources are sodium hydroxide and sodium acetate. Sources for thepotassium ion include potassium hydroxide and potassium acetate.Suitable nickel ion sources are nickel acetate, nickel oxide and nickelhydroxide. Sources of magnesium include magnesium oxide, magnesiumhydroxide and magnesium acetate. Sources of manganese include manganeseacetate and manganese oxide.

The metal cation neutralized high acid ionomer resins can be produced byreacting the high acid base copolymer with various amounts of the metalcation salts above the crystalline melting point of the copolymer, suchas at a temperature from about 200° F. to about 500° F., preferably fromabout 250° F. to about 350° F. under high shear conditions at a pressureof from about 10 psi to 10,000 psi. Other blending techniques may alsobe used. The amount of metal cation salt utilized to produce the newmetal cation neutralized high acid based ionomer resins is the quantitywhich provides a sufficient amount of the metal cations to neutralizethe desired percentage of the carboxylic acid groups in the high acidcopolymer. The extent of neutralization is generally from about 10% toabout 90%.

Examples of commercially available hard ionomeric resins which can beused in a blend to form the inner cover layer 14 include the hard sodiumionic copolymer sold under the trademark Surlyn®8940 and the hard zincionic copolymer sold under the trademark Surlyn®9910. Surlyn®8940 s acopolymer of ethylene with methacrylic acid with about 15 weight % acidwhich is about 29% neutralized with sodium ions. This resin has anaverage melt flow index of about 2.8. Surlyne®9910 is a copolymer ofethylene and methacrylic acid with about 15 weight % acid which is about58% neutralized with zinc ions. The average melt flow index ofSurlyn®9910 is about 0.7. Additional examples include lotek 4000, lotek4010, lotek 8000, lotek 8020, lotek 8030, lotek 7010 and lotek 7030which are sold by Exxon Corporation. It is to be understood that theabove examples are for illustrative purposes and are by no meansintended to be limiting.

Non-limiting examples of soft ionomers to be blended with theabove-described hard ionomers to form the inner cover layer 14 of theinventive golf ball comprise sodium, zinc magnesium, calcium, manganese,nickel, potassium or lithium, etc. salts of a terpolymer of an olefinhaving from about 2 to 8 carbon atoms, acrylic acid or methacrylic acid,and a softening comonomer which is an unsaturated monomer of theacrylate ester class having from 1 to 21 carbon atoms. The soft ionomeris preferably an ionomer made from an acrylic acid-based polymer and anunsaturated monomer of the acrylate ester class. The soft (low modulus)ionomers have a hardness from about 20 to 40 as measured on the Shore Dscale and a flexural modulus from about 1,000 to about 15,000, asmeasured in accordance with ASTM method D-790.

Certain ethylene-acrylic acid based soft ionomer resins developed by theExxon Corporation under the designation "lotek 7510" may be combinedwith known hard ionomers such as those indicated above to produce theinner cover. The combination produces higher CORs at equal or lowerlevels of hardness, higher melt flow (which corresponds to improved,more efficient molding, i.e., fewer rejects) as well as significant costsavings versus the outer layer of multi-layer balls produced by otherknown hard-soft ionomer blends as a result of the lower overall rawmaterials costs and improved yields. Furthermore, lotek 7520,Surlyn®8320 and Surlyn®8120 also can be used.

It has been determined that when hard-soft ionomer blends are used forthe inner cover layer, good results are achieved when the relativecombination is in a range of about 90 to about 10% hard ionomer andabout 10 to about 90% soft ionomer. The results are improved byadjusting the range to about 75 to 25% hard ionomer and 25 to 75% softionomer. Even better results are noted at relative ranges of about 60 to40% hard ionomer resin and about 40 to 60% soft ionomer resin.

Specific formulations which may be used in the cover composition areincluded in the examples set forth in U.S. Pat. No. 5,120,791 and4,884,814. The present invention is in no way limited to those examples.

The outer cover layer 16 of the golf ball is harder than the inner coverlayer and generally, but not necessarily, has a different chemicalcomposition than the inner cover layer. The outer cover layer 16preferably comprises a high acid ionomer resin or high acid ionomerblend in which more than 10 weight % of the acid groups are neutralized.Preferably, the outer cover layer 16 is formed from a blend of two ormore high acid ionomer resins that are neutralized to different extentsusing different metal cations. Softening modifiers can be used toachieve the desired Shore hardness of the outer cover layer.

The high acid ionomers which can be used in formulating the outer coverlayer of the golf ball are ionic copolymers which are the metal salts ofthe reaction product of an olefin having from about 2 to 8 carbon atomsand an unsaturated monocarboxylic acid having from about 3 to 8 carbonatoms. As nonlimiting examples, sodium, zinc, magnesium, manganese,potassium, calcium, nickel and lithium can be used as the neutralizingcation. Preferably, the ionomeric resins are copolymers of ethylene witheither acrylic or methacrylic acid. In some circumstances, an additionalcomonomer such as an acrylate ester, e.g. iso-n-butylacrylate, etc. canalso be used to produce a somewhat softer terpolymer. The carboxylicacid groups of the copolymer are partially neutralized by the metalions, i.e., about 10-75% or more preferably about 30-70%. Each of thehigh acid ionomer resins which may be included in the outer cover layercomposition preferably contains more than 16% by weight of a carboxylicacid, preferably 17-25% by weight of a carboxylic acid, and mostpreferably about 18.5-21.5% by weight of a carboxylic acid. Examples ofcommercially available high acid methacrylic acid-based ionomers whichcan be used in accordance with the invention include Surlyn® AD-8422(sodium cation), Surlyn® 8162 (zinc cation), Surlyn® SEP-503-1 (zinccation), and Surlyn® SEP-503-2 (magnesium cation). According to DuPont,all of these ionomers contain from about 18.5 to about 21% by weightmethacrylic acid.

Examples of high acid acrylic acid-based ionomers which can be used inthe present invention also include the Escor® or lotek High AcidEthylene lonomers produced by Exxon, including lotek Ex-959 and lotekEx-960. Furthermore, a number of other high acid ionomers neutralized tovarious extents by different types of metal cations, includingmanganese, lithium, potassium, calcium and nickel and several new highacid ionomers and/or high acid ionomer blends other than sodium, zincand magnesium are now available for golf ball cover production.

As indicated above, softening comonomers can be included in theionomeric inner and/or outer cover layers. Non-limiting examples of asoftening comonomer include vinyl esters of aliphatic carboxylic acidswherein the acids have 2 to 10 carbon atoms, vinyl ethers wherein thealkyl group contains 1 to 10 carbon atoms, and alkyl acrylates ormethacrylates wherein the alkyl group contains 1 to 10 carbon atoms.Suitable softening comonomers include vinyl acetate, methyl acrylate,methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate,butyl methacrylate, or the like.

Examples of a number of copolymers suitable for use to produce the highacid ionomers used in the present invention include, but are not limitedto, high acid embodiments of an ethylene/acrylic acid copolymer, anethylene/methacrylic acid copolymer, an ethylene/itaconic acidcopolymer, an ethylene/maleic acid copolymer, an ethylene/methacrylicacid/vinyl acetate copolymer, an ethylene/acrylic acid/vinyl alcoholcopolymer, etc. The base copolymer broadly contains greater than 16% byweight unsaturated carboxylic acid, from about 30 to about 83% by weightethylene and from 0 to about 40% by weight of a softening comonomer.Preferably, the copolymer contains about 20% by weight unsaturatedcarboxylic acid and about 80% by weight ethylene. Most preferably, thecopolymer contains about 20% acrylic acid with the remainder beingethylene.

As indicated above, the inner cover layer 14 and/or outer cover layer 16optionally may include hardening or softening modifiers, non-limitingexamples of which include a metal stearate, such as zinc stearate, oranother fatty acid salt, as described in commonly assigned U.S. Pat.Nos. 5,306,760 and 5,312,857. One purpose of the metal stearate or otherfatty acid salt is to reduce the cost of production of the ball withouteffecting overall performance of the finished ball. Furthermore,polar-group modified rubbers can be blended with ionomers as described,for example, in commonly assigned U.S. Pat. Nos. 4,986,545, 5,098,105,5,187,013, 5,330,837 and 5,338,610. Thermoplastic elastomers which actas hardening or softening agents, including polyurethane, a polyesterelastomer such as that sold by DuPont as Hytrel®, a polyesterpolyurethane such as B. F. Goodrich Company's Estane® polyesterpolyurethane X-4517, and a polyester amide such as that sold by ElfAtochem S. A. under the name Pebax®, can be added. A plastomer such asthat sold by Exxon under the name Exact™, e.g., Exact™ 4049 can beincluded. Various plasticizers and processing aids also can be used.

When the golf ball of the invention has more than two cover layers, theinner cover layer as defined in this application can be formed from twoor more layers which, taken together, meet the requirements of softness,thickness and compression of the layer or layers which are definedherein as the inner cover layer. Similarly, the outer cover layer can beformed from two or more layers which, taken together, meet therequirements of hardness, thickness and compression of the layer orlayers which are defined herein as the outer cover layer. Furthermore,one or more additional, very thin ionomeric or non-ionomeric layers canbe added on either side of the inner cover layer as long as theobjectives of the invention are achieved.

Having generally described the invention, the following examples areincluded for purposes of illustration so that the invention may be morereadily understood and are in no way intended to limit the scope of theinvention unless otherwise specifically indicated.

EXAMPLE 1

About 2 dozen thermoset polybutadiene golf ball cores having aconventional composition, a PGA compression of 93, a COR of 0.782 and adiameter of 1.47 inches were molded in a conventional manner. An innercover layer of a first ionomer formed from a blend of a soft acrylicacid based ionomer containing a softening comonomer and a hard acrylicacid based ionomer, designated as ionomer 1, having a thickness of 0.050inches and a Shore D hardness of 55 was injection molded over the cores.The resulting covered cores had a PGA compression of 94 and a COR of0.789.

An outer cover layer formed from a blend of two hard acrylic acid basedionomers and designated as ionomer 2 was injection molded over the innercover layer. The outer cover layer had a thickness of 0.055 inches and aShore D hardness of 68. The balls were primed and clear coated in auniform manner. The balls with the two-part cover had an average PGAcompression of about 99, and a COR of about 803. The spin rate of theballs was determined by striking the balls with a 9-iron underconditions of launch angle, ball speed and tee position which produced aspin rate of about 7100 rpm for a two-piece hard covered ball (1994Top-Flite XL) and a spin rate of about 9700 rpm for a thread woundbalata covered ball (1994 Titleist Tour 100) using the same club.

The properties of the balls, core and cover layers are shown on Table 1.

EXAMPLES 2-4

The procedure of Example 1 was repeated using three differentcombinations of inner and outer cover layer materials at the same coverlayer thicknesses. The results are shown on Table 1.

Comparative Examples 1-2

The procedure of Example 1 was repeated with the exception that the samematerial was used for the inner and outer cover layers of a particularball. In Comparative Example 1, two layers of ionomer 2 were used. InComparative Example 2, two layers of ionomer 4 were used. The resultsare shown on Table 1.

While the balls of Comparative Examples 1 and 2 exhibited favorable COR,their PGA compressions were unfavorably high. On the other hand, theballs of Examples 1-3 had favorable soft compressions while maintaininggood COR. The balls of example 4 had a reasonably good compression whilemaintaining a good COR. It is also noted that the balls of Examples 1-2had a somewhat higher spin rate than the balls of Comparative Example 1,and the balls Examples 3-4 had a somewhat higher spin rate than theballs of Comparative Example 2. This slightly higher spin willcontribute to improved playability and feel versus the balls ofComparative Examples while still being relatively low in spin ascompared to soft covered balls.

Comparative Examples 3-6

Conventional thermoset polybutadiene golf ball cores having a diameterof 1.57 inches, a PGA compression of 88, and a COR of 780 were coveredwith a single layer of cover material having a thickness of 0.055inches. Four different types of ionomeric covers were used. The ballswere then primed and clear coated in the same manner as the balls ofExamples 1-4. The resulting two-piece golf balls had measurements of PGAcompression, COR, Shore D cover hardness, and spin as shown on Table 1.

As shown in Comparative Examples 3-6, the COR of the golf ballsincreased as the hardness of the ball increased. As shown by comparativeExample 6, ionomer 1 is too soft for use alone as an outer cover layerand results in an insufficient COR. As shown by Comparative Example 5,ionomer 3 is also soft and produced a ball having a marginallyacceptable COR and a relatively hard compression. The balls ofComparative Examples 3 and 4 are inferior to the balls of Examples 1-4because for a given COR, the PGA compressions of the balls ofComparative Examples 3 and 4 are relatively hard, even though the corecompressions were softer (prior to cover molding) than those of Examples1-4.

EXAMPLES 5-6

The procedure of Example 1 was repeated with the exception that coreshaving sizes of 1.43 inches and 1.545 inches were used. Furthermore,inner and outer cover layers of different thicknesses were used. Thecover layer thicknesses and properties as well as resulting golf ballproperties are shown on Table 1.

COMPARATIVE EXAMPLE 7

The procedure of Example 5 was repeated with the exception that theinner and outer cover layers were formed from the same material. Theresults are shown on Table 1.

While Example 5 and Comparative Example 7 both had ionomeric coverlayers with the same overall thickness, the ball of Example 5 issuperior to that of Comparative Example 7 in that the compression of theball of Comparative Example 7 is too high. Again, the spin rate forExample 5 is somewhat higher than Comparative Example 7, which willcontribute to improved feel and playability while not being so high (aswith a soft-covered ball) as to cause problems (slice/hook) for the lessskilled golfer. In order to reduce compression, rather than reducing theouter cover layer thickness, as was done in Comparative Example 3,Example 5 shows that by using a multi-layer ionomer cover with a softerinner layer and a harder outer layer, a golf ball having a favorablecombination of relatively soft compression and good coefficient ofrestitution can be obtained.

As shown by Example 6, a slight reduction in the thickness of theoverall ionomeric cover material does not substantially affect thecompression of the ball and contributes to favorable values of COR.

                                      TABLE 1                                     __________________________________________________________________________    Core                                                                                       Hardness                                                                      (Shore D)                                                                              Inner Cover Layer                                       Example                                                                            Size                                                                              COMP                                                                              [if ready     Thickness                                                                          COMP.sup.2                                                                            Hardness                              #    (inches)                                                                          (PGA)                                                                             available]                                                                         COR.sup.1                                                                         Material                                                                           (inches)                                                                           (PGA)                                                                             COR.sup.1,3                                                                       (Shore D)                             __________________________________________________________________________    1            93                                                                                       Ionomer 12                                                                           0.050                                                                                  789                                                                                 55                              2            9347                                                                                     Ionomer 3                                                                             0.050                                                                                 795                                                                                 63                              Comp. 1                                                                              1.47                                                                                93                                                                                       Ionomer 22                                                                            0.050                                                                             94                                                                                799                                                                                 68                              3            9347                                                                                     Ionomer 182                                                                          0.050                                                                                  789                                                                                 55                              4            937                                                                                      Ionomer 382                                                                           0.050                                                                                 795                                                                                 63                              Comp. 2                                                                              1.47                                                                               93                                                                                        Ionomer 482                                                                          0.050                                                                                  805                                                                               73                                Comp. 3                                                                              1.57                                                                                88                                                                                         None  780                                                                         --                                                                              --  --  --                                    Comp. 4                                                                              1.57                                                                                88                                                                                         None  780                                                                           --- --  --                                    Comp. 5                                                                              1.57                                                                                88                                                                                         None 780                                                                            --                                                                            --  --  --                                    Comp. 6                                                                              1.57                                                                                88                                                                                         None 780                                                                            --                                                                            --  --  --                                    5           --.43                                                                                     Ionomer 17                                                                           0.090                                                                                  792                                                                                 56                              6           89.545                                                                                    Ionomer 10                                                                           0.033                                                                                  794                                                                                 56                              Comp. 7                                                                              1.43                                                                                    --                                                                                   Ionomer 2                                                                                 81503                                                                                   68                              __________________________________________________________________________                      Outer Cover Layer                                                                            Ball                                                      Example   Thickness                                                                          Hardness                                                                           COMP    SPIN                                              #    Material                                                                           (inches)                                                                           (Shore D)                                                                          (PGA)                                                                             COR.sup.1                                                                         (RPM)                                __________________________________________________________________________                 1              0.0552                                                                                       7508                                            2              0.0552                                                                                     809                                                                             7650                                            Comp. 1                                                                             Ionomer 2                                                                              0.055                                                                                      819                                                                             7390                                            3             0.055 4                                                                                     809                                                                             6626                                            4              0.055                                                                              73                                                                                    816                                                                             6358                                            Comp. 2                                                                              Ionomer 4                                                                           0.055                                                                                      830                                                                               6258                                            Comp. 3                                                                             Ionomer 2                                                                              0.055                                                                                      800     68                                                                      7331                                            Comp. 4                                                                              Ionomer 4                                                                             0.055                                                                                      808    73                                                                       6516                                            Comp. 5                                                                             Ionomer 3                                                                              0.055                                                                                        8421 63                                         Comp. 6                                                                              Ionomer 1                                                                             0.055                                                                                        9479 55                                         5             0.057 2                                                                                     799                                                                             7435                                            6             0.057 2                                                                                     809                                                                             7382                                            Comp. 7                                                                             Ionomer 2                                                                              0.057                                                                              68                                                                                    820                                                                             7230                               __________________________________________________________________________     .sup.1 Actual COR values have been multipled by 1000                          .sup.2 COMP of inner cover layer and core                                     .sup.3 COR of inner cover layer and core                                 

We claim:
 1. A golf ball, comprising:a core, an inner cover layer formeddirectly over the core, the inner cover layer substantially consistingof ionomer and having a thickness of 0.05-0.10 inches and a Shore Dhardness of 56-64, and an outer cover layer comprising an ionomer, theouter cover layer being harder than the inner cover layer and having aShore D hardness of at least about 60, the sum of the thickness of theinner cover layer and the thickness of the outer cover layer being atleast 0.090 inches.
 2. A golf ball according to claim 1, wherein theball has a coefficient of restitution of at least 0.780.
 3. A golf ballaccording to claim 1, wherein the outer cover layer has a thickness ofabout 0.025-0.200 inches.
 4. A golf ball according to claim 1, whereinthe inner cover layer has a Shore D hardness of 63 or less.
 5. A golfball according to claim 4, wherein the outer cover layer has a Shore Dhardness of at least
 65. 6. A golf ball according to claim 5, whereinthe core comprises a thermoset rubber.
 7. A golf ball according to claim1, wherein the inner cover layer comprises an ionomer having an averagewt % acid content of about 15 or less which is at least 10% neutralized.8. A golf ball according to claim 1, wherein the outer cover layercomprises an ionomer having an average wt % acid content of about 16 ormore that is at least 10% neutralized.
 9. A golf ball according to claim1, wherein the golf ball has a diameter of at least 1.70 inches.
 10. Agolf ball according to claim 1, wherein the core comprises a thermosetrubber.
 11. A golf ball according to claim 1, wherein the inner coverlayer comprises a blend of a hard ionomer and a soft ionomer.
 12. A golfball according to claim 1, wherein the PGA compression of the ball is nomore than about
 110. 13. A golf ball according to claim 1, wherein thePGA compression of the ball is no more than about
 100. 14. A golf ballaccording to claim 1, wherein the outer cover layer has a Shore Dhardness of about
 73. 15. A golf ball according to claim 1, wherein thegolf ball has no more than two cover layers.
 16. A golf ball accordingto claim 1, wherein the golf ball has a diameter of at least 1.70inches.
 17. A golf ball according to claim 1, wherein the outer coverlayer has a thickness of 0.050-0.057 inches.
 18. A golf ballcomprising:a core, an inner cover layer formed directly over the core,the inner cover laver substantially consisting of ionomer and having athickness of 0.05-0.10 inches and a Shore D hardness of 56-65, and anouter cover layer comprising an ionomer, having a Shore D hardness of atleast 60, and being harder than the inner cover layer, wherein the golfball has a PGA compression of no more than about 110 and a coefficientof restitution of at least 0.780, and the ratio of the diameter of theball to the sum of the thickness of the inner cover layer and thethickness of the outer cover layer is less than 19:1.
 19. A golf ballaccording to claim 18, wherein the golf ball has no more than two coverlayers.
 20. A method of making a multi-layer golf ball,comprising:obtaining a golf ball core, forming an inner cover layerdirectly over the core, the inner cover layer substantially consistingof ionomer and having a thickness of 0.05-0.10 inches and a Shore Dhardness of 56-65, and forming an outer cover layer comprising anionomer over the inner cover layer, the outer cover layer having a ShoreD hardness of at least about 60 and being harder than the inner coverlayer, the combined thickness of the inner cover layer and the outercover layer being at least 0.090 inches.
 21. A method according to claim20, wherein the core, inner cover layer and outer cover layer are formedsuch that the PGA compression of the ball is no more than about
 107. 22.A method according to claim 20, wherein the core, inner cover layer andouter cover layer are formed such that the ball has a coefficient ofrestitution of at least 0.780.
 23. A method according to claim 20,wherein the outer cover layer is applied in a thickness of about0.025-0.200 inches.
 24. A method according to claim 20, wherein thediameter of the core, and the thicknesses of the inner cover layer andouter cover layer are selected in order to form a golf ball having adiameter of at least 1.70 inches.
 25. A method according to claim 20,wherein the outer cover layer has a Shore D hardness of about
 73. 26. Amethod according to claim 20, wherein the outer cover layersubstantially consists of ionomer.
 27. A method according to claim 20,wherein the step of obtaining a core comprises obtaining a solid corecomprising a thermoset rubber.